The present invention broadly relates to an optical disk system having a head actuator adapted for driving the optical head in the radial direction of an optical disk and, more particularly, to a device for driving and controlling the optical head in such an optical disk system. Still more particularly, the present invention is concerned with a speed control circuit for conducting speed control of the head actuator during access to a track.
FIG. 1 is a block diagram of the control system in a known optical head driving and controlling device used in an optical disk system disclosed in Japanese Patent Application No. 156526/1986. Referring to FIG. 1, the optical disk system has an optical disk 1 provided with tracks in the form of concentric circles or a spiral which carry information. A light beam, which serves as an information transfer media between the optical disk 1 and an optical head 3, is denoted by 2. The optical head 3 is housed in a carriage 4 which is driven by a head actuator 5 so that it moves across the tracks on the optical disk 1. The carriage 4 is provided with a tracking actuator 6 provided with a collimator lens (not shown) which is adapted to focus the light beam 2 so as to form a spot on the track of the optical disk 1. The tracking actuator 6 is adapted to perform, after the spot of the light beam 2 has been positioned approximately on the aimed track by the operation of the head actuator, fine adjustment of the position of the spot and subsequent tracking control of the beam spot. A two-divided photo detector 7 has a pair of sensors and is adapted to convert light reflected on the optical disk 1 into electrical signals and output these signals. Each of the sensors outputs an electrical signal in response to the quantity of light in the light beam 2 introduced to the sensor. An addition circuit 8 adds the signals supplied from the two-divided photo detector 7 to each other and outputs a movement signal waveform in response to the transversal movement of the spot of the light beam 2. A subtraction circuit 9 performs subtraction between the signals supplied from the two-divided photo detector 7 so as to detect the deviation of the spot of the light beam 2 from the center of the track of the optical disk. A direction detection circuit 10 detects, on the basis of the outputs from the addition circuit 8 and the subtraction circuit 9, the direction in which the spot of the light beam moves. A speed detection circuit 11 detects, on the basis of the output signal from the subtraction circuit 9, the speed at which the spot of the light beam 2 crosses the tracks on the optical disk 1. A pulse generation circuit 12 generates a pulse from a signal supplied from the subtraction circuit 9 each time the spot of the light beam 2 crosses each track on the optical disk 1. A track counter 13 receives a signal supplied from the outside and corresponding to the number of access-tracks N, and counts and outputs the number of remaining tracks by a pulse signal supplied from the pulse generation circuit 12. A reference speed generation circuit 14 receives a signal which represents the number of remaining tracks and which is supplied from the track counter 13, first determines and stores a reference speed pattern corresponding to the number of remaining tracks, and successively outputs reference speed signals for operating the head actuator 5 in accordance with the change in the number of remaining tracks supplied from the track counter 13. A reference-speed-direction assignation circuit 15 assigns a direction D input from the outside to a reference speed signal which is output from the reference speed generation circuit 14. A speed-direction assignation circuit 16 assigns, from the output of the direction detection circuit 10, a direction of the speed of the spot of the light beam 2 detected by the speed detection circuit 11. A tracking servo pull-in command generator 17 receives the outputs from the subtraction circuit 9, the speed detection circuit 11 and the track counter 13, and outputs a position control command when the speed of the spot of the light beam 2 decreases to a predetermined speed at a predetermined track before the spot of the light beam reaches a target track. A tracking-servo circuit 18 drives the tracking actuator 6 on the basis of the outputs from the tracking servo pull-in command circuit 17 and the subtraction circuit 9 so as to control the fine adjustment in positioning the spot of the light beam 2, and the operation of following up the target track. A speed error detection circuit 19 detects the difference between the speeds determined by the reference speed signal, supplied from the reference-speed-direction assignation circuit 15, and by a moving speed signal, suppled from the speed-direction assignation circuit 16 and representing the speed at which the spot of the light beam 2 moves. An amplifier circuit 20 amplifies an output signal supplied from the speed error detection circuit 19 and outputs a control signal to the head actuator 5.
FIG 2 is a diagram of input-output characteristics of a conventional circuit corresponding to the reference speed generation circuit 14. FIG. 3 is a diagram of input-output characteristics of a conventional circuit corresponding to the speed detection circuit 11. FIG. 4 is a waveform diagram both of a reference speed output V.sub.REF from a circuit in a conventional speed control system corresponding to the reference-speed-direction assignation circuit 15, and of a speed detection output V.sub.SD from a circuit corresponding to the speed-direction assignation circuit 16.
The operation of this control system will be described below. During access to one of the tracks, the carriage 4, incorporating the optical head 3, is moved by the head actuator 5 so that it crosses the tracks on the opticals disk 1. A signal N supplied from the outside, and corresponding to the number of access-tracks is input into the track counter 13. When, during track-accessing, the spot of the light beam 2 crosses the tracks on the optical disk 1, electrical signals which correspond to this crossing movement are output from the sensors of the two-divided photo detector 7. These output signals are input into the addition circuit 8 and the subtraction circuit 9, thereby respectively obtaining summation signals, namely, REF signals (total reflection quantity signals) and differential signals, namely, tracking signals. The pulse generation circuit 12 generates pulses on the basis of a signal supplied from the subtraction circuit 9 in such a manner that it generates a pulse, for example, at a zero point of this signal at which each cycle of the signal starts. The thus generated pulses that represent the extent to which the spot of the light beam 2 has crossed the tracks are output to the track counter 13. The track counter 13 receives the pulse signal from the pulse generation circuit 12 and successively subtracts corresponding values from the number of access-tracks N first supplied from the outside, and outputs signals each of which represents the number of remaining tracks. The reference speed generation circuit 14 receives the output from the track counter 13, and determines and outputs a reference speed pattern signal in accordance with the number N.sub.A of remaining tracks (residual tracks) on the basis of the input-output characteristics shown in FIG 2. The reference-speed-direction assignation circuit 15 assigns to the output from the reference speed generation circuit 14 a reference-speed-direction command D which specifies the direction of the movement of the head actuator 5 and which has been input from the outside, thereby outputting a reference speed. The direction detection circuit 10 receives the output from the addition circuit 8, namely, the REF signal and the output from the subtraction circuit 9, namely, the tracking signal, and detects, from the phases of these signal waveforms, the direction in which the spot of the light beam 2 moves. The speed detection circuit 11 receives the output from the subtraction circuit 9 (tracking signal), detects the tracking signal cycle, converts it into a speed at which the spot of the light beam 2 moves across the track on the optical disk 1 in accordance with the characteristic shown in FIG. 3 (into a value in proportion to the reciprocal number of the period), and thereafter outputs this speed. The speed-direction assignation circuit 16 receives a speed signal output from the speed detection circuit 11 and a direction signal output from the direction detection circuit 10, and outputs a beam-spot-speed detection signal with a selected direction. A reference speed output from the reference-speed-direction assignation circuit 15 and the beam-spot-speed detection signal from the speed-direction assignation circuit 16 are then input into the speed error detection circuit 19 and are compared with each other therein. A differential signal thereby obtained is amplified by the amplifier circuit 20. The head actuator 5 is driven on the basis of the output from the amplifier circuit 20 so as to control the speed at which the carriage 4 is moved and, hence, the speed at which the spot of the beam 2 is moved. As shown in FIG. 4, as the spot of the light beam 2 approaches the target track, the speed at which the spot of the light beam 2 moves decreases by the control of the operation of driving the head actuator 5. The tracking servo pull-in command generator 17 receives output signals supplied from the subtraction circuit 9, the speed detection circuit 11 and the track counter 13, and outputs a position control command signal if the speed becomes lower than a predetermined value when the spot of the light beam reaches a predetermined track, for example, the track adjacent to the target track before the beam spot reaches the target track. The tracking-servo circuit 18 receives the output from the tracking servo pull-in command generator 17 and the output from the subtraction circuit 9 (tracking signal), and controls the tracking actuator 6 in the vicinity of the zero point of the tracking signal, thereafter proceeding to a positional control mode. That is, the light spot can be positioned at the target track.
If, in the thus constructed device for driving and controlling the optical head, some offsets occur in the electrical circuits including the speed detection circuit 11, the reference speed generation circuit 14, the reference-speed-direction assignation circuit 15, the speed-direction assignation circuit 16, the speed error detection circuit 19 and the amplifier circuit 20, this would affect the control of the operation of driving the head actuator 5, as indicated by the broken line in FIG. 4, and reduce the accuracy of the control. This is because the signal levels of the speed detection output and the reference speed output are small as the speed of the light spot is low immediately before pull-in, namely, the operation of positioning the spot at the target track. Therefore, there is a possibility of a reduction in the accuracy of the drive control by the head actuator 5 due to circuit-system offsets and, hence, a possibility of the tracking-servo circuit 18 failing to perform the pull-in operation at the target track, resulting in increase in the access time.